Vibration and Seismographs

One fact of life that we as commercial quarry blasters must accept is we at some point will need to deal with our neighbors in the community. Seismographs are a tool we rely heavily on to protect our neighbors. Our seismographs are equipped with electromagnetic sensors that measure ground motions and verify that vibrations are within standards set to protect nearby structures.

Years ago seismographs, much like computers, were not easily transportable. Many of the first portable seismographs were very heavy, built-in to a briefcase that had a printer included in the set-up. Today, we are lucky enough to have seismographs that can be all packed into a carrying case similar in size to a soft-sided lunchpail. This allows blasters to carry several seismographs and easily deploy them when previously it was a big deal just to set up one monitor.

Scaled Distance

Prior to the common use of seismographs in commercial blasting applications, blasters were held to limits based on a mathematic formula. This formula, know as Scaled Distance, is based on how many maximum pounds of explosives any given blast has detonating within any 8 ms interval. This formula was established based on a study performed by the US Bureau of Mines over 30 years ago. This study showed, based on a number of blasts performed with seismic monitors on nearby homes, that blasts were more likely to cause damage when large amounts of explosives were detonated within a certain time frame of one another. This makes pretty logical sense where you would expect to see more of an impact on a blast that has more pounds of explosives going on at once than one that doesn’t.

However, what this study also showed is if charges were separated by at least 8 ms, the magnitude of the impact would not be multiplied. Therefore if you have 100 holes with 100 lbs of explosives going off at once, you would expect to see more impact than a blast that has the 100 holes going off one hole at a time with 8 ms gap between each hole. This study also was part of understanding that delaying a blast where the holes went off in a chain, as opposed to all at once as originally thought, gave the operator the best fragmentation possible. This Scaled Distance formula gave blasters something to design their blasts around. If you were able to stay under a certain amount of explosives detonating within an 8 ms delay, you would not cause any damage to nearby structures.

Avoiding Damage to Structures

When we talk about “damage” to a structure, we are talking about very low levels of impact. The lowest standard level we are held to is that of dry wall. The limit for damaging a foundation, concrete, wells, or anything else is at least five times higher than that of the level where one could possibly see damage to dry wall. Therefor, it is a common practice in the blasting industry to adhere to the most stringent regulations. This means we base all our blasts around staying below the level where one could see damage to dry wall. By doing this, we are over five times less than vibration level which could cause damage to any foundations, wells, etc.

More recently, this 8 ms rule has been debunked as inaccurate, meaning it has been proven blasters can achieve lower vibration limits even when blasting large amounts of explosives within 8 ms of eachother. This is thanks to signature holes, which we can discuss in another post.

Seismographs Today

Now, if blasters are able to set up seismographs, the Scale Distance formula does not apply. Therefor, we are allowed more freedom in blast design and in conjunction with electronic detonators, we are able to lower vibration levels, increase fragmentation, and better control costs and efficiency in the drilling and blasting process.

Today, we own and operate several seismographs that are set up at blast sites daily. These seismographs have computer touch screens that are able to upload their measurements remotely instead of relying on a hard copy coming from a printer. With more and more structures being built closer and closer to quarries, seismographs are a necessity for us to do our job safely and efficiently every day. They have allowed us to blast in areas never before thought possible but thanks to the technology developed today we are able to complete projects in days that would have otherwise taken weeks or months.